It has become possible to utilize X-rays with high brightness, low emittance and high coherence in various wavelength ranges from soft X-rays to hard X-rays at 3rd-generation synchrotron radiation facilities represented by SPring-8. This resulted in a drastic improvement in sensitivity and spatial resolution of various analyses such as a fluorescent X-ray analysis, photoelectron spectroscopy, and X-ray diffraction. These X-ray analyses and X-ray microscopy utilizing a synchrotron radiation not merely provide high sensitivity and high resolution, but also make non-destructive observations possible, and are therefore currently being used in the fields of medicine, biology, material science, and the like.
In 3rd-generation synchrotron radiation facilities, 3.5th-generation synchrotron radiation facilities many of which are already under construction or in operation, or X-ray free electron laser facilities which are currently being under construction, a highly focused X-ray nanobeam is required in order to provide high spatial resolution with various analysis techniques utilizing an X-ray. A group of the inventors of the present invention has already succeeded in focusing a hard X-ray with a wavelength of 0.6 Å so as to have a focused beam diameter of 30 nm or less by using a light focusing optical system which is composed of a Kirkpatrick and Baez (K-B) mirror at the 1 km-long beam line of SPring-8. This success is due in large part to a high-precision mirror processing technique and high-precision mirror shape measuring techniques which have been uniquely developed. This processing technique refers to a numerically controlled elastic emission machining (EEM) a process principle of which is such that a high shear flow of ultrapure water mixed with fine particles is formed along a surface of a mirror to be processed, the fine particles are combined together with atoms on the surface of the mirror by a kind of chemical reaction, and the atoms on the surface are removed with movement of the fine particles. Further, the shape measuring techniques refer to a microstitching interferometry (MSI) and a relative angle determinable stitching interferometry (RADSI) a measurement principle of each of which is such that pieces of partial shape data taken by an interferometer which is capable of high precision shape measurement of small areas are put together to thereby obtain the entire shape data. The use of these shape measuring techniques makes it possible to measure the shape of an X-ray mirror with a high degree of accuracy in all spatial wavelength ranges with a measurement reproducibility of 1 nm or less (PV).
In order to achieve hard X-ray focusing with a smaller focused beam diameter and high energy from here on, it is necessary to manufacture a mirror having a large curvature and a shape with higher accuracy. Accordingly, it becomes essential to improve the performance of a shape measuring instrument. However, even if a shape measurement utilizing the above described nanometrology techniques (MSI and RADSI) is carried out with high accuracy and nanomachining (EEM) is performed based on the obtained shape data to thereby achieve nano-order accuracy in the shape of a reflective surface of a mirror, a wavelength of a reference light of the measuring instrument and a wavelength of an X-ray at the time of focusing generally differ significantly between when the shape of the focusing mirror is measured and when the mirror is actually used in an X-ray focusing device. In addition, the shape of the reflective surface is strained in a subtle way due to temperature or other installed environmental conditions, thereby affecting the focusing performance. In order to achieve the most ideal focusing at diffraction limit, it is necessary to know the shape of the reflective surface of the focusing mirror in a state of being incorporated in the X-ray focusing device with high accuracy. Therefore, the inventors have proposed an at-wavelength metrology in which a phase error in a mirror surface is calculated by phase retrieval calculation only from X-ray intensity profile information in a light focusing surface, and also proposed an X-ray focusing method in which a phase error of a light focusing optical system is corrected based on the phase error in the mirror surface calculated in the above metrology to thereby eliminate irregularities in a wavefront of a focal plane (Patent Document 1). Further, in order to accurately calculate a phase error of an X-ray mirror by the phase retrieval method, it is essential to acquire a precise focused X-ray beam intensity profile. The inventors have therefore proposed a new method for accurate measurement of an X-ray nanobeam intensity distribution that utilizes a dark-field method using a knife edge (Patent Document 2).
Further, in Patent Document 1, there has been proposed the use of a reflective surface shape controllable mirror having a wavefront adjustable function that enables a fine adjustment of a wavefront of an X-ray. Patent Document 1 discloses the specific structure of the reflective surface shape controllable mirror in which a mirror surface layer which has a reflective surface formed thereon and is elastically deformable is stacked on a base having high shape stability with a deformation drive layer therebetween. In the deformation drive layer, a common electrode layer is formed on one surface of a piezoelectric element layer and a plurality of divided drive electrode layers are formed on the other surface. A controlled voltage is applied between the common electrode layer and each of the drive electrode layers from driver means, a specific area of the sandwiched piezoelectric element layer is thereby deformed, and the deformation causes a change in the shape of the mirror surface layer.
Further, Patent Document 3 discloses a bimorph mirror which is capable of changing the surface shape. The bimorph mirror includes first and second layers of piezoelectric ceramic together with at least one electrode and serves to change at least one curvature of the mirror in response to at least one voltage applied to the piezoelectric ceramics. The first and second layers of piezoelectric ceramic are separated by a central core which forms a semirigid beam and is composed of a material such as glass or silica. Further, the first and second layers of piezoelectric ceramic are sandwiched between two skin layers which are composed of glass, silicon or the like, wherein at least one of the skin layers is for use as a mirror.
However, in bimorph type reflective surface shape controllable mirrors described in Patent Document 1 and Patent Document 3 mentioned above, since the thermal expansion coefficient of the piezoelectric element which is used for allowing the surface shape to be deformable is different from that of the material of the mirror (quartz, silicon, or the like), the mirror shape is sensitively distorted under the influence of a temperature difference. Generally, when manufacturing a nano-focusing K-B mirror, the surface shape nano-measurement (MSI and RADSI) and EEM are carried out by repetition in order to bring the mirror to completion. In this case, since EEM is performed in fluid, the surface shape is distorted due to a difference between the temperature at the time of measurement and the temperature at the time of machining. As a result, the distortion of the mirror generated between the measurement time and the machining time causes a big problem in achieving nm-order shape accuracy. For example, in the case of a bimorph mirror in which the material of the mirror is quartz and a piezoelectric element used therein is made of ceramic, since the mirror has a layered structure with materials having different thermal expansion coefficients, the surface shape varies by approximately 5 to 10 nm between 9 and 70 hours after EEM is performed on the mirror, as shown in FIG. 13. Further, it is impossible to actually match the temperature at the time of focusing operation to the temperature at the time of mirror machining. Therefore, even if the mirror is fabricated with nano-level shape accuracy, the surface shape of the mirror is distorted during a focusing operation due to a temperature difference, thereby causing a large shape error.